Knee up

Having a knee-up means having a fun-filled party bustling with music, dancing and excitement.

When using as a noun, a hyphen is needed in between “knee” and “up to make it a compound word “knee-up”.

It’s a countable noun with plural form “knees-up” (not knee-ups).

Etymology 

This phrase is believed to have originated from a British pub song Knees Up Mother Brown written by Bert Lee in London.